I guess that some people fancy the idea of having a guard cat and believe that in order to be effective the animal has to be much larger than existing domestic cats. Is it possible to achieve this?
You can’t achieve this objective unless you domesticate a much larger wild cat species than the Near Eastern wildcat (North African wildcat). The reason why the domestic cat is the size that it is, is because it’s a domesticated Near Eastern wildcat. There is a limit to how large you can selectively breed such a cat. The well-known Maine Coon is the largest domestic cat breed unless you include the first filial wild cat hybrids such as the F1 Savannah cat.
The F1 Savannah cat is genuinely quite a large cat as you can tell from the video (Magic was and perhaps still is the tallest domestic cat in the world). It is certainly large enough to behave as a guard dog. The father is a serval and the mother is a domestic cat. The serval is quite a large wild cat species as you might know.
If you want to go any larger than an F1 Savannah cat you would have to domesticate wild cat species such as the cheetah, puma or leopard. It is my contention that these cats cannot genuinely behave as domesticated animals and as pets although you will see the occasional person who has a domesticated mountain lion.
There are also domesticated servals in America but again I have to say it doesn’t really work unless you want your home sprayed with urine and a cat who will do anything to escape from what is effectively a captive lifestyle in the home or in an enclosure in the garden.
To return, therefore, to the question in the title, it is not possible to genuinely breed domestic cats as big as large dogs because the wild cat species from whence they would have to come do not have the personalities to be domesticated successfully.
The reason why the domestic cat is successful is because the wild cat ancestor, is quite a pliable, affable species suited to domestication.
The nearest you can get to meet your objectives in compliance with the question in the title is the F1 Savannah cat which already exists. However, being domesticated they’re not meant to be guard animals. It would seem that dogs are more suited to being guard animals whereas, perhaps, if you trained an F1 Savannah cat to be a guard animal it might tend to be aggressive to both strangers and to their owners because cats do not see their owners as masters as dogs do. Dogs see their owners as pack leaders whereas cats don’t.
The answer therefore is probably NO.